The present invention relates generally to power conversion systems and methods, and more particularly, to a circuit arrangement for recovering the energy stored in the input capacitance of a field effect transistor that is used to turn it on.
There is a continuing need to improve the efficiency of electronic equipment. This need comes from the continued size reduction of electronic equipment being accomplished through high level integration and hybrid technologies. In the field of power conversion there also exists an increasing demand for smaller more efficient power conversion methods. To improve the density of power converters it is necessary to increase the conversion frequency. Since one of the main contributors to low efficiency in low voltage (1 to 5 volts) power converters is the rectifier loss, FETs are the most likely candidates for use as both a control element and as the rectification element in high frequency density power converters. FETs contribute a loss that is caused by the charging of its input capacitance and by an ohmic conduction loss. The charging loss increases directly with frequency and with the physical die size of the FET. A large die size is required to minimize the ohmic conduction losses caused by the FET.
The present state of the art typically removes the energy stored in the input capacitance of a FET by discharging the capacitance in a resistive element. The energy is lost as heat. A paper published by staff members of Virginia Polytechnic Institute and State University has described a means of resonantly charging the input capacitance of FETs to improve turn on. This paper is entitled "Zero-Voltage-Switched Quasi-Resonant Buck and Flyback Converters - Experimental Results at 10 MHz", authored by W. A. Tabisz, Gradzki, and F. C. Lee, 1987 IEEE Proceedings of the Power Electronics Specialists Conference. A NASA Tech Brief, Vol. 14, No. 5, entitled "Recovering Energy From a Rapidly Switched Gate also discussed efficiently charging and discharging gate capacitance. However, these references do not discuss recovery of the energy stored in FET input capacitance at turn-off.
More specifically, there is an ever increasing need in radar and communication systems and in automotive applications to improve the performance while reducing the size and weight and improving the efficiency of power conversion equipment. This is conventionally accomplished in part by operating power conversion equipment at higher operating frequencies than is typical of conventional equipment. It is therefore an objective of the present invention to by providing a means of recovering the energy stored in the input capacitance of a power transistor by its turn-on signal.